The present invention relates to coated composites containing silicon carbide whiskers distributed in an alumina based matrix, and which have an alumina coating thereon. It especially relates to metalcutting inserts made from such coated composites.
In the prior art, a variety of materials have been utilized for machining of ferrous and nonferrous metals. Among the prior art materials that have been utilized for the machining of steels have been both the coated and uncoated cemented carbides. Specific coated cemented carbides are described in U.S. Pat. No. 4,610,931 and are exemplified by the prior art commercial KC950 grade which is made in accordance with the foregoing patent (KC950 is a Trademark of Kennametal Inc. for its coated cutting inserts).
In addition, steels have also been machined utilizing TiC-TiN cermets such as those described in U.S. Pat. No. 3,971,656. Steels have also been commercially machined using ceramics such as alumina and alumina-titanium carbide ceramics. A prior art example of an alumina-titanium carbide cutting insert grade is the K090 grade, which contains about 30 volume percent titanium carbide (K090 is a Trademark of Kennametal Inc. for its alumina-titanium carbide cutting inserts).
While the foregoing compositions have been successful in machining steels, the cemented carbides are commercialy utilized under a combination of high feeds and low speeds, whereas the ceramic grades are utilized under a combination of high speeds and low feeds. We are not aware of any commercially available cutting tool grade which has been commercially utilized to machine steel at a combination of high speeds and high feeds.
Cutting inserts made of alumina-silicon carbide whisker composites (see U.S. Pat. No. 4,543,345) have been developed. These inserts, while commercially useful in the machining of nickel base superalloys, have not been practically applied to the machining of steels due to the poor tool lifetimes obtained. It is believed that these poor tool lifetimes are due to a strong chemical reaction which occurs between the iron in the steel and the silicon carbide in the cutting insert at the high temperatures produced at the cutting edge during machining.
The prior art commercial grades of alumina-silicon carbide whisker composites utilized in the machining of nickel base alloys have contained between 30 to about 36 v/o silicon carbide whiskers, since it has been found that machining performance is optimized in this composition range. Maximum fracture toughness is also achieved at this level of SiC whiskers.